Active sodium (Na+) transport across the adult alveolar epithelium plays an important role in the maintenance of lung fluid balance, especially after sublethal hyperoxic injury to the blood-gas barrier, when the effectiveness of the passive Starling forces is diminished. Presently, the mechanisms by which Na+ ions enter the apical membranes of normal and oxygen-injured alveolar epithelial cells, have not been elucidated. Based on preliminary data, we hypothesize that alveolar type II cells (ATII) contain Na+ channels with low affinity to amiloride and that the properties and spatial distribution of these channels may be altered by exposure to sublethal hyperoxia. Since sodium channels conduct at rates far exceeding that of any other transporter, and their activities may be upregulated by a number of agents, they may form a major pathway for the entry of Na+ ions into alveolar epithelial cells. The overall goal of this research project is to assess the distribution of these ion channels in the alveolar epithelium of normal and hyperoxic-injured rats, characterize their properties at the single cell level and study some of their fundamental regulatory mechanisms. Sublethal hyperoxic injury will be induced by exposing rats to 60 h of 100% 02 and returning them to room air for 24 h and 72 h, an exposure period known to increase the activity of lung Na+-K+ ATPase, and the rate of fluid removal across the alveolar epithelium. These studies will be conducted in both freshly isolated and cultured ATII cells to assess changes in the properties of these channels with time in culture. The specific aims of this application are: (1) to define the selectivity, single ion current, open and close time probabilities, sensitivity to amiloride and other pharmacological agents by patch-clamp techniques; (2) to establish the existence of amiloride-binding protein(s) in ATII cells by Western blotting techniques and determine their spatial localization by immunocytochemical studies at both the light and electron microscopic level; and (3) to investigate whether phosphorylation of these channels via the cAMP-dependent protein kinase (PKA) correlated with increased Na+ transport at both the whole cell and single channel level. Completion of these specific aims will provide new and fundamental knowledge on the possible mechanisms of fluid clearance across the alveolar space of both the normal and injured mammalian lung.